FIG. 1 shows a conventional optical interferometric gyro. Light emitted from a light source 11 is split by a beam splitter 12 into two, one of which is provided via a polarizer 13 to an optical coupler 14 and the other of which is terminated at a terminating element 20. The light beam led to the optical coupler 14 is split into clockwise and counterclockwise light beams. The clockwise light beam is phase modulated by a phase modulator 15 immediately after being emitted from the optical coupler 14, and is provided to one end of an optical fiber coil 16 used as a looped optical transmission line. The clockwise light beam reaches the optical coupler 14 again after propagating through the optical fiber coil 16 clockwise. On the other hand, the counterclockwise light beam is provided to the other end of the optical fiber coil 16 and, after propagating therethrough counterclockwise, it is phase modulated by the phase modulator 15, immediately thereafter reaching the optical coupler 14 again. In the optical coupler 14 the clockwise and counterclockwise light beams having propagated through the optical fiber coil 16 meet and interfere with each other. At this time, a periodic phase difference occurs between the clockwise and counterclockwise light beams because they have been subjected to periodic phase shifts by the phase modulator 15. Now, assume that the frequency f.sub.m of the modulation signal for driving the phase modulator 15 is, for example, 1/(2.tau.) (where .tau. is the time for light to propagate through the optical fiber coil 16). In this instance, when the phase shift of the clockwise light beam is sinusoidal as shown in FIG. 2A when it has just returned to the optical coupler 14 after being subjected to a phase shift .phi..sub.cw by the phase modulator and then having propagated through the optical fiber coil 16, a phase shift .phi..sub.ccw to which the counterclockwise light beam is subjected in the phase modulator 15 lags behind the modulation signal of FIG. 2A by the time .tau., and hence is 180.degree. out-of-phase with the phase shift .phi..sub.cw as depicted in FIG. 2B. Accordingly, the phase difference .phi..sub.cw -.phi..sub.ccw between the clockwise and counterclockwise light beams, which are combined by the optical coupler 14, varies with a 2.tau. period as indicated by the curve 17 in FIG. 2. In consequence, the two light beams, which are combined into the interference light, strengthen and weaken each other repeatedly with a period .tau., that is, the interference light varies its intensity with the period .tau.. The intensity of the interference light varies with the phase difference .phi..sub.cw -.phi..sub.ccw between the two light beams as indicated by the curve 18 in FIG. 2, and consequently, the intensity variation is repeated with the period .tau. as indicated by the curve 19.
In FIG. 1 the interference light from the optical coupler 14 is provided via the polarizer 13 to the beam splitter 12, wherein it is split into two beams, one of which is converted by a photodetector 21 into an electrical signal. This electrical signal becomes a signal which varies at a frequency twice higher than the phase modulation frequency f.sub.m, i.e. 1/.tau. in the example of FIG. 2.
When an angular velocity is applied to the optical fiber coil 16 about its axis, a phase difference corresponding to the input angular velocity is introduced between the clockwise and counterclockwise light beams by the Sagnac effect. As a result of this, the phase difference based on the input angular velocity is superimposed on the curve 17 in FIG. 2. When the phase difference is superimposed on the curve DC-wise, a component of the phase modulation frequency f.sub.m appears in the output electrical signal of the photodetector 21 in accordance with the DC-wise phase difference. The output of the photodetector 21 is provided to a synchronous detector 22, wherein it is synchronously detected by a reference signal of the same frequency as the phase modulation frequency f.sub.m. When the input angular velocity is zero, the output of the photodetector 21 is only an even multiple component of the phase modulation frequency, mainly a twice component alone, and consequently, the output of the synchronous detector 22 is zero. When an angular velocity is input, a component of the same frequency as the phase modulation frequency f.sub.m is provided at the output of the photodetector 21, and an output of the polarity and level corresponding to the direction and magnitude of the input angular velocity is obtained from the synchronous detector 22 and is provided to an output terminal 23; thus, the input angular velocity can be detected. The phase modulation signal to be supplied to the phase modulator 15 and the reference signal to be supplied to the synchronous detector 22 are produced by a modulation signal generator 24.
When passing through the phase modulator 15, the light is subjected to intensity modulation as well as to phase shift by the modulation signal. The reason for this is as follows: The phase modulation is performed by changing the refractive index of the medium through which the light propagates, but when the refractive index of the medium varies, the confinement of light in the medium varies accordingly, so that the confinement of light in the medium varies in synchronism with the phase modulation signal and the intensity of light passing through the phase modulator 15 is modulated in synchronism with it.
Thus, the clockwise and counterclockwise light beams, which have passed through the phase modulator 15, are both intensity-modulated at the frequency f.sub.m, and the interference light into which the thus intensity-modulated-clockwise and counterclockwise light beams have been combined again by the optical coupler 14 also contains the intensity-modulated component of the frequency f.sub.m. On this account, even when the input angular velocity is zero, the component of the frequency f.sub.m is detected from the synchronous detector 22 and this becomes an offset error of a bias value of the optical interferometric gyro. When the offset error is large, zero stability deteriorates because if any factor changes owing to an external disturbance or the like, the zero point will vary at a fixed rate.
As described above, when the input angular velocity is zero, the interference light is intensity modulated at the frequency twice higher than that f.sub.m of the modulation signal, and the interference light is split by the beam splitter 12 into two, one of which is supplied to the photodetector 21 and the other of which returns to the light source 11. Based on the interference light having thus returned to the light source 11, the light to be emitted therefrom is intensity modulated at the frequency twice that f.sub.m of the modulation signal. Alternatively, the interference light having thus returned to the light source is detected by a photo diode which is provided to control the quantity of light to be emitted from the light source 11, and an automatic quantity-of-light stabilizer operates to keep constant the quantity of light from the light source, including the detected output, so that the light from the light source 11 is intensity modulated at the frequency twice that f.sub.m of the modulation signal.
When the light from the light source 11, thus intensity modulated at the frequency 2f.sub.m, is intensity modulated by the phase modulation signal in the phase modulator 15, it is intensity modulated at frequencies 2f.sub.m +f.sub.m =3f.sub.m and 2f.sub.m -f.sub.m =f.sub.m by a frequency mixing effect of the modulation wave in the phase modulator 15. Thus, the interference light, into which the two light beams having passed through the phase modulator 15 are combined, also contains the intensity-modulated component of the frequency f.sub.m, and as described previously, even if the input angular velocity is zero, the synchronous detector 22 yields an output, which becomes an offset error of the bias value.
It is an object of the present invention to provide an optical interferometric gyro in which even if intensity modulation by the phase modulator is present and even if the light from the light source is intensity modulated, no error is induced in the bias value and hence the bias zero stability can be enhanced accordingly.
Moreover, in the prior art the branching ratio of the beam splitter 12 for splitting the light emitted from the light source 11 is set to 1:1. Assuming that the loss by optical elements is zero and the quantity of light incident from the light source 11 is 100, the quantity of light which is branched to each of the polarizer 13 and the terminating element 20 is 50, and when the quantity of light 50 is split again by the beam splitter 12 after having propagated through the optical fiber coil 16, the quantity of light returning to the light source 11 and the quantity of light of the signal to the photodetector 21 are each 25. In this case, the signal-to-noise ratio of the optical system can be set to a large value, but the quantity of light returning to the light source 11 also becomes maximal, leading to the defect of deteriorated-performance of the optical interferometric gyro.
The light source 11 for use in the optical interferometric gyro is, in many cases, a light source which employs an optical resonator, such as a semiconductor laser. Usually, the semiconductor laser constitutes an optical resonator, using cleavage planes at both ends of the laser chip as reflectors, and utilizes, as laser light, light having resonated in the resonator. In the light source which utilizes the resonance of light, such as the semiconductor laser, if light reflected or returning from a portion other than the cleavage planes of the semiconductor laser chip is incident to the laser chip, another resonator will be formed in addition to that one formed by the semiconductor laser. This resonator is formed outside the semiconductor laser, and hence is called an external resonator. When the external resonator is formed, the spectral configuration, center wavelength and coherence of the light source undergo changes. It has been reported that such a phenomenon is caused also in a super luminescent diode (SLD) which is often used in the optical interferometric gyro.
The variation in the spectral configuration, center wavelength and coherence of the light source 11 are fatal to the optical interferometric gyro, and the variation in the center wavelength is particularly serious because it is related directly to a change in the scale factor of the optical interferometric gyro. The scale factor of the optical interferometric gyro is a function of the center wavelength of the light source 11, and hence the variation in the center wavelength leads to a change in the scale factor. Furthermore, the variation in the coherence of the light source 11 also poses a serious problem because it causes a change in the value of the bias error which is induced by reflected or scattered light in the optical interferometric gyro.
It is therefore another object of the present invention to provide an optical interferometric gyro which suppresses the variation in the center wavelength of the light source which causes a change in the scale factor and induces a bias error and the light returning to the light source which causes a variation in the coherence of the light source, and hence is small in the scale factor variation and in the bias error.